Electro-optical apparatus for precise on-line measurement of the thickness of moving strip material

ABSTRACT

On-line apparatus for monitoring and determining the instantaneous thickness of a moving longitudinal strip of planar material simultaneously at several locations along the lateral cross-section thereof. The apparatus is particularly useful where the material is of non-uniform lateral cross-section and in addition is subject to flopping and undulatory movement in its travel past the monitoring station. The electro-optical system utilizes a pair of laser light sources, with associated beam splitter and lens systems, for focusing a plurality of spots of light of well-defined small diameter on the respective major surfaces of said material at locations along the lateral crosssection thereof, and then projecting the reflected images of said respective light spots onto corresponding detector units. Each of the detector units is in the form of a plurality of individual photo-sensors arranged in masked array to generate a binary-coded electrical output signal, the numerical significance of which is indicative of the displacement of the light image from a nominal position and thus is representative of the distance deviation of the surface of said material at each cross-section location from a predetermined reference plane. Arithmetic and logic elements next operate on the respective digital output signals from said detector units to derive resultant numerical measurements of the instantaneous thickness of said moving material at each of said lateral locations. In cases where one of the major surfaces of the moving material is constrained to a fixed plane, for example by having its bottom surface in contact with a moving conveyor bed, then distance deviation measurements need only be made at lateral locations on the other, unconstrained major surface of the material to determine the instantaneous thickness of the material at each of said locations.



1. Apparatus for monitoring and determining the instantaneous position of a surface of a moving strip of material comprising: a. means for projecting a spot of light on the surface of said material, b. means for gathering reflected light from said spot formed on said surface and projecting a real image of said spot on a plane, and c. photodetection means for determining the position of said image on said plane relative to a predetermined point on said plane, said photodetection means providing a quantified electrical signal output indicative of said image position and representative of the instantaneous relative position of said strip surface at the cross-sectional location where said light spot is incident thereon.
 2. The apparatus set forth in claim 1 characterized in that said electrical signal output of said photodetection means is in the form of a digital number numerically representative of said image position, and further including (d) digital computer means with associated arithmetic and logic circuitry for converting said output signal from said photodetection means into a digital signal numerically representative of the instantaneous thickness of said strip at said cross-sectional location.
 3. The apparatus of claim 1 adapted for an application where said strip is not constrained to a fixed plane of travel and characterized in that a pair of devices each comprised of means (a), (b) and (c) are arranged at a cross-sectional location on said strip for determining the position of respective opposed surfaces thereof in order to provide output signals which are indicative of the instantaneous thickness of said strip at said cross-sectional location.
 4. The apparatus of claim 3 further characterized in that the instantaneous thickness of said strip is measured at a plurality of locations along the cross-section thereof by providing a respective pair of devices comprised of means (a), (b) and (c) at each said location, and that means (a) is comprised of a single light source with associated beam-splitting prisms for providing all of the light spots projected on a respective surface of said strip.
 5. The apparatus of claim 4 in which there are n number of cross-sectional locations on said strip at which the instantaneous thickness is to be measured and characterized in that the reflectivity of a beam-splitting prism associated with a location which is a number j away from said light source is substantially in accordance with the formula:
 6. Apparatus for monitoring and determining the instantaneous position of a major surface of a moving longitudinal strip of planar material which is subject to variations in thickness comprising: a. light source means for projecting a spot of light of small diameter, on the order of 5 millimeters or less, on a major surface of said material, b. image-forming means for gathering reflected light from said spot formed on said surface and projecting a real image of said spot on a plane, said means including a pickup lens and a cylindrical defocusing lens for spreading out the image of said light spot into a narrow band of light, and c. photodetection means situated in said image plane for determining the position of said image relative to a predetermined point on said plane, said photodetection means comprising a plurality of individual photo-sensor cells arranged in an array for generating a binary-coded electrical output signal which changes in digital increments in response to displacement of the position of said image across the face of said photodetection means, whereby said digital output signal from said photodetection means is representative of the positional deviation of said surface of said strip at said cross-sectional location from a predetermined reference plane.
 7. The apparatus set forth in claim 6 characterized in that said array of photo-sensor cells in said photodetection means is adapted to produce a Gray-coded digital signal output, and conversion means are provided to transform said Gray-coded signal into a conventional binary-coded signal.
 8. The apparatus set forth in claim 6 adapted for an application where said strip is not constrained to a fixed plane of travel and characterized in that a pair of devices, each comprised of light source means (a), image-forming means (b) and photodetection means (c), are arranged at a cross-sectional location on said strip for determining the position of respective major surfaces thereof, and the digital output signals produced by the respective photodetection means are supplied to digital computer means having associated arithmetic and logic circuitry for converting said respective photodetection output signals into a digital signal numerically representative of the instantaneous thickness of said strip at said cross-sectional location.
 9. The apparatus of claim 8 further characterized in that the instantaneous thickness of said strip is measured at a plurality of locations along the cross-section thereof by providing a respective pair of devices comprised of means (a), (b) and (c) at each said location, and that light source means (a) is comprised of a laser with associated beam-splitting prisms for providing all of the light spots projected on a respective major surface of said strip.
 10. The apparatus of claim 8 further characterized in that said digital computer means compares said signal representative of the instantaneous thickness of said strip at said cross-sectional location with predetermined minimum and maximum limits and generates an actuating signal in the event the measured thickness value falls outside said limits.
 11. The apparatus of claim 6 characterized in that, in situations where the angle ( theta ) between the incident light from said light source means (a) forming the spot on said surface of said strip and the reflected light from said spot gathered by said image-forming means (b) is not 90*, then the plane on which said image of said spot is formed is tilted by an appropriate angle ( Beta ) with respect to the normal to the optical axis of said pickup and cylindrical lens system of said image-forming means to compensate for the defocusing effect which would ordinarily be produced on said image by the variation in object-to-image distance in said image-forming lens system as said spot moves in response to positional displacement of said strip surface.
 12. The apparatus of claim 11 further characterized in that the angle theta is substantially 45*, and the angle of tilt Beta of the image plane is substantially in accordance with the expression: tan Beta M, where M is the degree of magnification of said image-forming lens system.
 13. The apparatus of claim 6 further characterized in that said light source means (a) includes a light chopper for producing a pulsating component to said projected light spot.
 14. Apparatus for monitoring and determining the position of an object comprising: a. means for projecting a spot of light on said object, b. means for gathering reflected light from said spot formed on said object and projecting a real image of said spot on a plane, and c. photodetection means for determining the relative position of said image on said plane, said photodetection means comprising a plurality of individual photo-sensor cells arranged axially in said plane and each generating a respective electrical signal when said light spot image is incident thereon whereby the resultant output of said photodetection means is indicative of said image position and representative of the relative position of said object at the point where said light spot is incident thereon.
 15. The apparatus set forth in claim 14 further including means enabling said photodetection means to distingUish said reflected light from ambient background light.
 16. The apparatus set forth in claim 14 wherein said light-projection means includes a laser producing a continuous-wave coherent light beam.
 17. The apparatus set forth in claim 14 wherein said projected light is directed against said object so as to be incident at substantially right angles to the surface thereof. 